In a cellular communication system, signaling and voice or data communications between mobile communication units and fixed communication units (such as the infrastructure which is composed of cells) are carried over an RF channel. In a CDMA system such as described by IS-95, IS-2000, or WCDMA, the RF channel is an RF carrier with signals from many communications independently spread with the use of orthogonal Walsh codes and other unique identifiers. In a TDMA system such as GSM, the RF channel is an RF carrier and timeslot combination. As a mobile unit moves through a cellular system, it becomes necessary to support a handoff of the mobile unit from the first fixed communication unit to a second fixed communication unit. In the case of CDMA, this handoff might be a soft handoff that allows the first fixed communication unit to remain connected with the mobile unit along with the second fixed communication unit. Soft handoff then permits cell site diversity to improve the communication paths by combining the best signals from each fixed communication unit connection.
Cellular systems have always been faced with the challenge of reducing or eliminating dropped calls. However, in an interference-limited environment, when the traffic channel degrades, the ability for the mobile and the infrastructure to exchange signaling messages for the purpose of handover is severely hampered. If this message exchange cannot take place, the necessary handover cannot occur to move the mobile out of its current degraded traffic channel condition. This ultimately results in a dropped call.
A partial solution to the dropped call problem is described in U.S. Pat. No. 5,913,167, METHOD FOR TRANSFERRING A COMMUNICATION LINK IN A WIRELESS COMMUNICATION SYSTEM, by Bonta, et al. The '167 patent provides the mobile with a list of reserved channels that can be used for handoff from the first fixed communication unit to the second fixed communication unit when communication between the mobile unit and a first fixed communication unit is interrupted. Although the '167 patent provides a partial solution to this problem, no mechanism exists to insure that the mobile unit and the first fixed communication unit are time coordinated to perform what amounts to a blind handoff. If the mobile unit and the infrastructure are not time-coordinated, the mobile unit may attempt to perform a handoff to infrastructure equipment that is unprepared to accept the handoff. In addition, the prior art does not provide a mechanism for handling simultaneous communication breakdowns that would lead to simultaneous dropped calls. This is particularly a problem when trying to efficiently utilize equipment resources.
Another proposed solution to the dropped call problem is provided in U.S. Pat. No. 6,337,983, Method for Autonomous Handoff in a Wireless Communication System, by Bonta, et al., which is assigned to the same assignee as the current invention. Generally, the Bonta application provides a method and apparatus for efficiently performing an autonomous handoff within a wireless communication system that reduces the chances that the infrastructure will be unprepared to accept the handoff when it occurs. More particularly, the Bonta invention utilizes a rescue channel in a handoff procedure that alleviates the need to send and receive handoff control messages. In the preferred embodiment of the Bonta invention, coordination of the rescue procedure at the infrastructure and mobile station is provided first by disabling the mobile transmitter and subsequently detecting signal loss and frame erasures at the serving cells, second by enabling the mobile transmitter and subsequently detecting signals by a rescue cell, and finally by enabling the rescue cell channel transmitter and subsequently detecting and receiving signals and frames by the mobile.
The Bonta invention however does not address the signaling between infrastructure components that must occur to implement the rescue channel procedure. Thus, there is a need for a signaling scheme between the communication system components to ensure that the infrastructure can signal the proper behavior to candidate rescue cells.